1. Field of the Invention
The present invention relates to a magnetic toner used for image forming methods such as an electrophotographic method, an electrostatic printing method, a magnetic recording method, and a toner jet method. Particularly, the present invention relates to a magnetic toner for developing electrostatic latent images, and an image forming method and a process cartridge, both of which use the magnetic toner.
2. Description of the Related Art
Many electrophotographic methods have been conventionally known, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 (corresponding to U.S. Pat. No. 3,666,363) and Japanese Patent Publication No. 43-24748 (corresponding to U.S. Pat. No. 4,071,361). In these methods, generally, an electric latent image is formed on a photosensitive member by any one of various means using a photoconductive material, and then the latent image is developed by using toner to form a toner image as a visible image, and if required, the toner image is transferred to a transfer material such as paper or the like, followed by fixing under heating, pressure or heating and pressure to obtain a copy or print.
There are also various known developing methods of visualizing electrostatic latent images by using toner. Examples of such developing methods include the magnetic brush method disclosed in U.S. Pat. No. 2,874,063, the cascade developing method disclosed in U.S. Pat. No. 2,618,552, and the powder cloud method disclosed in U.S. Pat. No. 2,221,776, a fur brush developing method, a liquid developing method, etc. Of these developing methods, particularly, the magnetic brush method, the cascade method and the liquid developing method, all of which use a two-component type developer mainly comprising a toner and a carrier, are brought into practical use. Although all these methods are excellent methods capable of relatively stably obtaining good images, they have a problem with respect to the two-component type developer in which the carrier deteriorates, and the mixing ratio of the toner and the carrier varies.
In order to solve the problems, various developing methods are proposed, which use a one-component type developer comprising only a toner. Particularly, methods using a one-component type developer comprising toner particles having magnetism are excellent.
U.S. Pat. No. 3,909,258 proposes a developing method using a magnetic toner having electric conductivity for development. In this method, a conductive magnetic toner is supported on a cylindrical conductive sleeve having magnetism therein, and is brought into contact with an electrostatic latent image holding member having an electrostatic latent image to develop the latent image. At this time, in the development unit, a conductive path is formed by toner particles between the surface of the electrostatic latent image holding member and the sleeve surface. Charge is led to the toner particles from the sleeve through the conductive path, and the toner particles are adhered to the image region due to Coulomb's force between the image region of the electrostatic latent image and the magnetic toner particles to develop the latent image. Although this method using a conductive magnetic toner is an excellent method capable of solving the problems of the conventional two-component developing methods, the method has a problem in which since the toner is conductive, it is difficult to electrostatically transfer a toner image from the electrostatic latent image holding member having the toner image to a final support member such as plain paper or the like.
As a developing method using a high-resistance magnetic toner which can be electrostatically transferred, there is a developing method which employs dielectric polarization of toner particles. However, this method has a problem in which the development speed is fundamentally low, and thus a developed image having a sufficient density cannot be obtained.
Another known development method using a high-resistance insulating magnetic toner is a method in which magnetic toner particles are triboelectrically charged by friction between the respective magnetic toner particles and friction between the magnetic toner particles and a triboelectric charging member such as a sleeve or the like to develop an electrostatic latent image by the magnetic toner having triboelectric charge. However, such a method has problems in which the number of times of friction between the magnetic toner particles and the triboelectric charging member is small, causing insufficient triboelectric charge, and in which the charged magnetic toner particles easily agglomerate on the sleeve due to an increase in Coulomb's force between the toner particles and the sleeve.
Japanese Patent Laid-Open No. 55-18656 discloses a new jumping development method capable of solving the above problems. In this method, a magnetic toner is thinly coated on a sleeve, and frictionally charged, and then the magnetic toner layer on the sleeve is brought near an electrostatic latent image to develop the latent image. In this method, the magnetic toner is thinly coated on the sleeve to increase the opportunity of contact between the sleeve and the magnetic toner, thereby permitting sufficient triboelectric charge. Also the magnetic toner is supported by magnetic force, and a magnet and the magnetic toner are relatively moved to prevent agglomeration of the magnetic toner particles and cause sufficient friction with the sleeve, thereby obtaining an excellent image.
The insulating toner used in the above development method comprises a sufficient amount of finely powdered magnetic material mixed and dispersed therein, and partially exposed from the surfaces of the toner particles. Therefore, the type of the magnetic material used influences the fluidity and triboelectric chargeability of the magnetic toner, thereby influencing various characteristics required for the magnetic toner, such as the development performance and durability of the magnetic toner, etc.
In further detail, in the conventional jumping development method using a magnetic toner containing a magnetic material, repetition of a development step (for example, copying) for a long period of time causes deterioration in the fluidity of a one-component type developer containing the magnetic toner, insufficient triboelectric charge, nonuniformity in charging, and fogging in an environment of low temperature and low humidity, thereby causing a problem of image quality. With low adhesion between the binder resin and the magnetic material which constitute the magnetic toner particles, repetition of the development step causes separation of the magnetic material from the surfaces of the magnetic toner particles. There is thus the tendency to cause an adverse effect on the toner image, such as a decrease in density of the toner image.
With the magnetic toner containing the magnetic material dispersed therein with nonuniformity, the small particles of the magnetic toner containing a large amount of magnetic material are accumulated on the sleeve, thereby sometimes causing a decrease in image density and the occurrence of nonuniformity in density, which is referred to as "sleeve ghost".
With respect to the magnetic iron oxide contained in conventional magnetic toner, magnetic toner containing magnetic iron oxide particles containing a silicon element is proposed in Japanese Patent Laid-Open Nos. 62-279352 (corresponding to U.S. Pat. No. 4820603), and 62-278131 (corresponding to U.S. Pat. No. 4975214). Although a silicon element is positively contained in such magnetic iron oxide particles, the magnetic toner containing the magnetic ion oxide particles has the need to improve the fluidity of the magnetic toner.
In Japanese Patent Laid-Open No. 3-9045 (corresponding to European Patent Application Publication EP-A187434), it is proposed that the shape of magnetic iron oxide particles is controlled to a spherical shape by adding silicate. In the magnetic iron oxide particles obtained by this method, large amounts of silicon element are distributed in the magnetic ion oxide particles because of the use of silicate for controlling the particle size, but the silicon element is less present on the surfaces of the magnetic iron oxide particles, thereby causing insufficient improvement in the fluidity of the magnetic toner.
A method is proposed in Japanese Patent Laid-Open No. 61-34070, in which triiron tetraoxide is produced by adding a hydroxosilicate solution in oxidization to triiron tetraoxide. The triiron tetraoxide particles obtained by this method contain Si element in the vicinity of the surfaces thereof, but have a problem in which the surfaces have low resistance to mechanical shock such as fraction or the like because the Si element is present in a layer near the surfaces of the triiron tetraoxide particles.
In Japanese Patent Laid-Open No. 5-72801, a magnetic toner is proposed, which contains magnetic iron oxide particles containing 0.4 to 4% by weight of silicon element, 44 to 84% of the total content of the silicon element being present in the vicinity of the surfaces of the magnetic particles.
In the magnetic toner containing the magnetic iron oxide particles, the fluidity of the toner, and the adhesion between the binder resin and the magnetic iron oxide particles are improved. However, in the magnetic iron oxide particles disclosed in a production example, a large amount of silicate component is present in the uppermost surfaces, and a porous structure is formed in the surfaces of the magnetic iron oxide particles, thereby increasing the BET specific surface area of the magnetic iron oxide particles. Therefore, the magnetic toner containing the magnetic iron oxide particles have the tendency that triboelectric charge properties deteriorate after allowing to stand in an environment of high humidity for a long time.
Japanese Patent Laid-Open No. 4-362954 (corresponding to European Patent Application Publication No. EP-A468525) discloses magnetic iron oxide particles containing silicon and aluminum elements. However, there is demand for further improving environmental properties.
Japanese Patent Laid-Open No. 5-213620 discloses magnetic iron oxide particles containing a silicon component which is exposed from the surfaces thereof. However, like the above magnetic iron oxide particles, there is demand for further improving environmental properties.
Japanese Patent Laid-Open No. 7-239571 discloses that magnetic iron oxide particles contain silicon element, and the Fe--Si ratio of the uppermost surface is controlled. Although this improves frictional chargeability in an environment of high humidity, the magnetic iron oxide particles described in a production example have the tendency that the bulk density is increased, and a toner containing the magnetic iron oxide particles is liable to be densely packed in a development unit.
In a high-capacity system in which the toner fill in the development unit is increased for complying with recent increases in processing speed and lifetime, such a magnetic toner is easily packed in the development unit due to the weight of the toner and the pressure by a agitator, thereby causing insufficient supply of the toner to the sleeve and a fading phenomenon in which an image is blanked in a strip.
Such a magnetic toner is also insufficient in improvement of fluidity. Particularly, when a cartridge is transported for a long time, the toner contained in the cartridge is deviated to one side and tapped therein. Therefore, in this state, image formation easily causes nonuniformity in distribution of the toner on the sleeve, and sometimes causes blanking in an image.
Japanese Patent Laid-Open Nos. 9-59024 and 9-59025 disclose magnetite particles containing 1.7 to 4.5 atomic % of silicon in terms of Si based on Fe, and, as a metal element other than Fe, 0 to 10 atomic % of at least one metal element selected from Mn, Zn, Ni, Cu, Al, and Ti based on Fe.
Although this can improve magnetic properties and chargeability, the fluidity of the toner cannot be sufficiently improved only by adding the above metals, and the toner has a property to be further improved.
Furthermore, in order to improve the fluidity of a toner, besides the magnetic material, other raw materials of the toner are demanded to be controlled for improving the fluidity.
Japanese Patent Laid-Open Nos. 62-226260, 63-139365, 3-50559 and 6-208244 disclose a toner or toner resin composition containing polypropylene modified by carboxylic acid or maleic acid. However, the fluidity of the toner cannot be sufficiently improved.
In recent years, there have been demand for increasing the operation speed and lifetime of an image forming apparatus using an electrophotographic technology, such as a copying machine and a laser beam printer, and demand for improving definition and quality of the toner image obtained. A toner and a process cartridge containing a toner are stored in a variety of environments, and thus storage stability is an important property of the toner.
In recent, as printer apparatus, light-emitting diode (LED) printer and laser beam printer have been mainly put on the market. In a technical tendency, resolution have been increased, i.e., conventional resolution of 240 or 300 dpi has been increased to 400, 600, or 1200 dpi. Accordingly, a development system has been required to have higher definition.
A copying machine is increasingly made highly functional, and is thus increasingly digitized. In this tendency, an electrostatic image is mainly formed by a laser, and thus resolution is also increased. In this case, like a printer, a development system having high resolution and high definition is required. Japanese Patent Laid-Open Nos. 1-112253 and 2-284158 disclose a toner having a small particle size.
However, a high-resolution and high-definition image can be formed by decreasing the particle size of a toner, while the surface area per unit weight of the magnetic toner is increased to increase the tribo charge of the magnetic toner. Therefore, the fluidity of the magnetic toner deteriorates, thereby making further significant the fading phenomenon and nonuniformity in the magnetic toner on the sleeve.